1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for transmitting/receiving an ultrasonic wave to obtain Doppler shift data of the ultrasonic wave in an object to be examined, obtaining movement data in the object by performing frequency analysis of the data, and using the movement data for a display.
2. Description of the Related Art
A conventional ultrasonic diagnostic apparatus often uses an ultrasonic transducer array constituted by a plurality of ultrasonic transducer elements.
In linear electronic scan, a predetermined number of ultrasonic transducer elements selected from a plurality of ultrasonic transducer elements constituting an ultrasonic transducer array are excited as a group, and an ultrasonic beam is transmitted. These ultrasonic transducer elements constituting one group are repeatedly excited in a pulse-like manner, while simultaneously and sequentially changing the selection of the ultrasonic transducer elements of that group in units of one element, thus electronically and sequentially shifting the transmission position of an ultrasonic beam to be transmitted. In linear electronic scan, the scanning of an object to be examined with an ultrasonic beam is done by using such a sequential shift of the transmission position of the ultrasonic beam. An ultrasonic beam transmitted from the group of ultrasonic transducer elements can be focused by creating phase differences between the transmitted sound waves from respective elements. This is obtained by shifting the time of excitation of the elements depending on their lateral positions relative to the beam, i.e., beam axial positions versus beam peripheral positions. Similarly, an ultrasonic beam received by the group of ultrasonic transducer elements can be focused in effect by utilizing phase differences between the signals received by the respective elements. This is obtained by shifting the delay times applied to the signals in reception depending on the lateral positions of the receiving elements, i.e., beam axial positions versus beam peripheral positions. Such a technique of focusing an ultrasonic beam is called electronic focusing. In the above-described linear electronic scan, electronic focusing is used where needed. An ultrasonic wave reflected in an object to be examined, i.e., an ultrasonic echo, is received and converted into an electrical signal by the same ultrasonic transducers as were used for transmission. For example, a tomographic image is formed on the basis of echo data obtained by such transmission/reception of the ultrasonic beam, and is displayed on a TV (television) monitor or the like.
In sector electronic scan, a plurality of ultrasonic transducer elements constituting an ultrasonic transducer array are repeatedly excited as a group to transmit an ultrasonic beam in a pulse-like manner. The ultrasonic beam transmitted from these ultrasonic transducer elements can be steered by creating phase differences between sound waves from the respective elements. These are produced by gradually shifting the excitation timing of respective elements. (Similarly, regarding an ultrasonic beam to be received by the ultrasonic transducer elements, directivity can be provided to the reception sensitivity in effect by utilizing phase differences between the signals received by the respective elements. This is achieved by applying a different delay time to each of the receiving elements so as to align the respective reception signals.) By sequentially changing the excitation timing of each transducer element so as to sequentially steer the direction of the ultrasonic beam in units of pulses, a sectorial region in the object is scanned with the ultrasonic beam. When such sector electronic scan is to be performed, the above-mentioned electronic focusing is employed where needed. Received echo data is processed in basically the same manner as in the above-described linear electronic scan.
In addition to electronic scan such as the above-described linear and sector electron scan, mechanical scan is available as a technique of scan using an ultrasonic beam. In mechanical scan, a transducer is mounted on a scan mechanism, and an ultrasonic beam is moved by operating the scan mechanism.
An ultrasonic Doppler method is, typically, a method of obtaining data on the basis of movement of blood (blood flow) in a living body and visualizing it. The ultrasonic Doppler method is based on the Doppler effect that when an ultrasonic wave is reflected by a moving matter, the frequency of the reflected wave is shifted in proportion to the moving velocity of the matter. More specifically, an ultrasonic wave as a pulse is transmitted into a living body, and the phase or frequency of an ultrasonic echo as the reflected wave of the ultrasonic pulse is detected by using the transmission ultrasonic wave as a reference, thereby obtaining frequency shift data based on the Doppler effect obtained by the movement of a matter which reflected the ultrasonic wave, i.e., Doppler shift data. Movement data of the matter causing reflection of the echo can be obtained from the Doppler shift data. According to this ultrasonic Doppler method, for example, the directions of blood flows, the velocities of blood flows, and states of changes in blood flow at various positions in a living body can be detected by analyzing the Doppler shift data.
A conventional ultrasonic diagnostic apparatus for obtaining blood flow data from an ultrasonic echo by using such an ultrasonic Doppler method will be described below.
An ultrasonic transducer is driven by a transmitting/receiving circuit to repeatedly transmit a pulse-like ultrasonic beam a predetermined number of times in a given direction. A received ultrasonic echo is detected by a phase detector, and a frequency data signal is obtained. This frequency data signal includes a Doppler shifted signal and a clutter component as an unwanted low-frequency component. The frequency data signal is converted into a digital signal by an A/D converter (analog-to-digital) converter. The clutter component is then removed by a digital filter. The Doppler shifted signal is frequency-analyzed by a high-speed frequency analyzer such as a moving-target indicator (MTI) processor using an autocorrelation scheme or the like. Upon frequency analysis of the Doppler shifted signal, for example, a mean Doppler shift, a Doppler shift variance, and a mean level of the Doppler shifted signal are obtained. Data such as a blood flow velocity, a blood flow velocity variance, and the power of the Doppler shifted signal can be obtained on the basis of the obtained values.
By performing the above-described processing while performing, e.g., sector scan of the ultrasonic beam, two-dimensionally distributed blood flow data similar to a B-mode tomographic image obtained by sector scan can be obtained. The two-dimensional blood flow data (e.g., a two-dimensional blood flow velocity image representing the direction and velocity of the blood flow) obtained in this manner, is superposed/combined on/with a B-mode image; alternatively, an image representing changes in blood flow data over time is superposed/combined on/with an M-mode image, thus displaying an ultrasonic image including the blood flow data on a monitor.
In such a conventional ultrasonic diagnostic apparatus, a clutter component is removed from a phase-detected ultrasonic echo signal by using the digital filter so as to extract only a Doppler shifted signal of a blood flow. A power calculator for obtaining blood flow power includes a blanking circuit for blanking noise contained in the Doppler signal with a blank level corresponding to predetermined power level. Since the noise which is contained in the Doppler signal and which has power equal to or less than the blank level is blanked, noise displayed on the screen of a TV monitor can be reduced to improve the image quality.
In the blanking circuit of the conventional apparatus, the following problem is posed.
As shown in FIG. 1, a phase-detected ultrasonic echo signal is filtered by a digital filter having a given cutoff frequency so as to extract only a Doppler shifted signal D distributed near a frequency 1/2 PRF which is 1/2 a pulse rate frequency (PRF), i.e., an ultrasonic pulse repetitive frequency and to remove a clutter component C. The conventional blanking circuit has a fixed blank level BL. For this reason, if the cutoff frequency of the digital filter is set to be relatively low frequency f1, a passband bw1 of the digital filter becomes wide, and increased noise corresponding to the wide band bw1 is caused. If the power of this noise N1 exceeds the blank level BL, a noise component (N1-BL) exceeding the blank level BL appears on an ultrasonic image including blood flow data, thus degrading the image quality.
The blank level has an effect on the integral value of the energy which the noise of each frequency has after passing through a digital filter. For simplicity, the blank level and noise level shown in FIG. 1 are depicted such that they are constant for all spectra.